a = [111, 2, undefined, 33];
console.log(a.sort()); // [ 111, 2, 33, undefined ]
// 原地排序
console.log(a); // [ 111, 2, 33, undefined ]
b = [111, 2, 33];
// 从小到大排序
console.log(b.sort((a, b) => a - b)); // [2,33,111]
